1. Field of the Invention
This invention relates to circuits for monitoring the internal resistance of a power source, which power source is of the type having a changing internal resistance as a function of the energy depletion thereof and, more particularly, to a monitoring circuit for monitoring the internal resistance of a lithium type battery as used in an implantable pacer for determining pacer end of life.
2. Description of the Prior Art
Over the last several years, the lithium-iodine battery has been widely adopted as a power source for the pacemaker industry, as well as other applications. Indeed, the broad use of this battery system in the pacemaker industry has resulted in it becoming substantially the standard power source for that industry. Well over 100,000 such batteries have already been implanted, but the lifetime of such cells is great enough that the industry has not yet experienced the first occurrence where such a battery has come to end of life under nominal load and normal circumstances. For this reason, there is not yet any actual experience with the end-of-life (EOL) characteristics of the lithium-iodine cell. However, methods have been developed for approximately the EOL curve of such batteries, and it has become evident that there is a great need for matching, or interfacing, the device being powered by the battery with the parameters of battery behavior, in order to optimize EOL operation.
In the practice of the invention of this application, reference is made to "lithium systems", meaning lithium-type battery cells. As pointed out in the article of Parsonnet et al, American Heart Journal, October 1977, Vol. 94, No. 4, pp. 517-528, there are at least 5 types of lithium systems in widespread use today, including lithium iodine types such as made by Wilson Greatbatch, Ltd. and Catalyst Research Corporation. This invention is directed particularly, but withou limitation, to the lithium systems characterized by having an internal resistance characteristic curve which is substantially linear as a function of energy depletion until near EOL, at which time the characteristic exhibits a knee and internal resistance rises rapidly. This characteristic of lithium type sources is discussed in my U.S. Pat. No. 4,031,899, which patent is incorporated herein by reference. In the lithium iodine type battery, the cell cathode consists of molecular iodine weakly bonded to polyvinyl pyridine (P2VP). At beginning of life, there are about 6 molecules of iodine to each molecule of P2VP. No electrolyte as such is included in construction of the cell, but a lithium iodine (LiI) electrolyte layer forms during cell discharge, between the anode and cathode. The LiI layer presents an effective internal resistance to Li+ ions which travel through it. Since the LiI layer grows with the charge drawn from the battery, or milliamp hours (mAh), this component of the battery resistance increases linearly as a function of mAh (i.e., as a function of cell energy depletion). In the pacemaker environment, since there is constant energy depletion, this component of the internal resistance increases continually with time. However, particularly for a demand pacer which at any time may or may not be delivering stimulus pulses, the increase of this component is not linear with time, due to the fact that current drain is not constant.
For the lithium iodine type cell, there is another component of internal resistance which is caused by depletion of iodine in the cathode. The cathode is essentially a charge transfer complex of iodine and P2VP, and during discharge of the cell iodine is extracted from this complex. In the beginning there are about 6 molecules of iodine to each molecule of P2VP. During extraction of iodine from the complex the resistance to this procedure is low until the point is reached where about only 2 molecules of iodine are left for each molecule of P2VP, at which point this resistance rises very sharply. This gives rise to a non-linear internal resistance component which, for the lithium system, is called variously the depletion resistance, the depolarizer resistance, the charge transfer complex resistance, or the pyridine resistance. By whatever names, the combination of the non-linear component with the linear component produces the resistance characteristic with a knee occurring toward EOL, the knee being caused by the reaching of depletion of available charge carriers from the cathode.
Although EOL has not yet become a problem in the sense that implanted lithium pacers are still too young to have exhausted their lithium battery sources, the pacer industry has started to become aware of the potential problem of determining EOL. Since the internal resistance of the source rises drastically after the knee, the battery has very little useful lifetime left after the knee has been reached. Some pacer manufacturers have predicated their design for determining EOL upon detection of the battery output voltage, which voltage comprises the constant open circuit voltage minus the drop caused by the current drain across the internal resistance. However, this is a highly tenuous and very unsatisfactory premise for determining EOL, due to the fact that actual current drain near EOL cannot be predicted. For any manufacturer's pacer which is implanted, and used in combination with a given electrode, there will be a variation in the effective load as seen by the lithium battery, and a resulting variation in the overall current drain. Accordingly, if the EOL design is predicated upon sensing voltage drop to a given level, there can be very little assurance that the level chosen will correspond to the knee of the cell curve.
One lithium cell manufacturer, Catalyst Research Corporation, in a paper presented to the Workshop on Reliability Technology For Cardiac Pacemakers, October, 1977, pointed out that for such batteries sensing of the battery internal resistance is more reliable than voltage sensing. The position that cell resistance rather than cell voltage is a better warning indicator is based upon the observation that the resistance characteristic has a much less steep EOL curve. Stated differently, at low currents typical for pacers, plots of resistance against time give more warning than plots of voltage against time. If voltage characteristics for different current drains are plotted, the knees are observed to have a fairly wide variation, meaning that the voltage at which the knee might appear is subject to substantial variation as a function not only of the particular battery being used but also the load being drawn by the pacer. On the other hand, plots of resistance indicate that the knee varies over a smaller range of values of internal resistance. Since the current drain may vary by as much as a factor of 2 due to different electrode loads, the variation in voltage may be twice as great as the variation of internal resistance. Monitoring the internal resistance provides a direct indication of the state of the battery, whereas monitoring the output voltage gives only a secondary indication, reflecting both the state of the battery and the operating condition of the pacer. This condition, it is anticipated, will be even more emphasized with the development of new, thin, large area cells which generally have steeper EOL slopes.
In my issued U.S. Pat. No. 4,031,899, there has been disclosed a circuit which can be programmed to provide an indication of the internal resistance of the lithium battery cell. As described in that patent, a switching circuit is utilized which alternately connects the relatively high current drain output circuitry and the relatively low current drain remainder of the circuit. By adjusting the duty cycle of the switching oscillator which controls the switching function, the voltage transferred from the battery to the respective circuits may be programmed to be substantially constant up until the resistance reaches the predicted value correspondingly to the knee of the curve, after which there is a programmed drop. Since the oscillator rate is linear as a function of delivered voltage, a programmed drop in frequency can be utilized to indicate that the battery resistance has reached the level where the knee was expected. This feature provides a decided advantage over EOL designs which sense voltage. However, the invention disclosed in this application has the added advantage that the exact resistance value at the knee need not be anticipated ahead of time. Even though the depletion resistance component is very predictable, the total value of the internal resistance at the time the knee is reached will be subject to some statistical variation so that EOL cannot be entirely accurately predicted by simply monitoring total battery internal resistance. What is acutely needed in the pacer industry, and provided for here, is circitry which accurately interfaces with the battery so as to make a highly reliable reading of the occurrence of the knee of the battery, along with means for providing an indication of the detection of such knee. The same need exists in other industries, such as electronic computers and space vehicles where batteries are utilized to maintain power during shut off of normal power, or in emergencies.